1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to an apparatus of driving a lamp and a liquid crystal display device using the same that provide a uniform brightness in the device.
2. Discussion of the Related Art
LCD devices have been actively developed as flat display panels in laptop computers, desktop computers, and large-sized information displays because of their high quality image, lightness, thin thickness, compact size, and low power consumption. Most LCDs are passive devices in which images are displayed by controlling an amount of light input from an outside light source. Thus, a separate light source (back light unit) is generally employed for irradiating an LCD device.
In general, a halogen cathode fluorescent tube or a cold cathode fluorescent lamp (hereinafter, referred to as a “CCFL”) is used as the light source in the back light unit. The CCFL is a light source tube using a cold emission phenomenon (electrons are emitted due to a strong electric field applied to the surface of a cathode) and has a low heat generation, a high brightness, and a long life span. The CCFL is classified into a light guide system, a direct illumination system and a reflection plate system, and an appropriate light source tube system among them is adopted in accordance with the requirement of LCD device.
In addition, the light source is classified into an internal electrode fluorescent lamp in which electrodes are formed inside of a discharge tube and an external electrode fluorescent lamp in which the electrodes are formed inside of the discharge tube. The fluorescent lamps are driven by a high-voltage alternating-current waveform. For example, after a direct-current (DC) power source supplied from a direct-current power source part is converted into an alternating-current waveform by an inverter, the high-voltage alternating-current (AC) waveform for driving the fluorescent lamps is boosted by a transformer.
FIG. 1 is a schematic view of an apparatus of driving a lamp according to the related art. In FIG. 1, an apparatus of driving a lamp includes lamps 24 for generating light and an inverter board 12 for driving the lamps 24. Each of the lamps 24 includes a glass tube having inert gases filled therein, and a first electrode and a second electrode at the opposite ends of the glass tube. An inner wall of the glass tube is coated with phosphor. When an AC waveform from the inverter board 12 is applied to the first and the second electrodes, electrons are emitted. These emitted electrons collide with the inert gases contained in the glass tube, and the number of electrons exponentially grows. These increased electrons generate electric currents in the glass tube, and excite the inert gases to emit ultraviolet rays. These ultraviolet rays collide with the phosphor coated on the inner wall of the glass tube, and generate visible lights.
The inverter board 12 generates a high-voltage AC waveform and supply the high-voltage AC waveform via a high-voltage line to first and second electrodes of the lamps 24. In particular, the inverter board 12 includes a plurality of inverters 10. Each of the inverters 10 includes an inverter integrated circuit converting a DC power source supplied from an exterior into an AC power source, and a transformer converting the AC power source from the inverter integrated circuit into the high-voltage AC waveform. Then, the high-voltage AC waveforms are outputted via a plurality of output channels CH1 to CH4 to the lamps 24.
For example, the first and the second channels CH1 and CH2 supply the high-voltage AC waveform to the first and the second electrodes of a first lamp 24. The third and the fourth channels CH3 and CH4 also supply the high-voltage AC waveform to the first and the second electrodes of a second lamp 24. A first high-voltage line 22 connected to the first electrode of the lamps 24 is connected to each of the first and the fourth output channels CH1 and CH4, and a second high-voltage line 20 connected to the second electrode of the lamps 24 is connected to each of the second and the third output channels CH2 and CH3.
Because a length of the first high-voltage line 22 is larger than a length of the second high-voltage line 20, an impedance difference between the first high-voltage line 22 and the second high-voltage line 20 is generated. Thus, a deviation of currents and voltages outputted form the inverter board 12 occurs due to the impedance difference between the first and the second high-voltage lines 22 and 20. As a result, a deviation in panel brightness is generated, to thereby induce an uneven brightness phenomenon in the panel.
FIG. 2 is a graph showing a current deviation caused by a high-voltage line shown in FIG. 1, and FIG. 3 is a graph showing a voltage deviation caused by the high-voltage line shown in FIG. 1. FIG. 2 is a measurement of the currents flowing on the high-voltage lines 22 and 20 at a measurement location 30 apart by about 5 cm from the output channels CH1 to CH4 (shown in FIG. 1). As shown in FIG. 2, a current value of about 70 mA is measured on the first high-voltage line 22 connected to the first and the fourth output channels CH1 and CH4 having a relatively long length. A current value of about 60 mA is measured on the second high-voltage line 20 connected to the second and the third output channels CH2 and CH3 having a relatively short length. Although not shown, this current deviation increases between the high-voltage lines 20 and 22 increases as they locate further from the output channels CH1 to CH4 and closer to the lamps 24.
Further, as shown in FIG. 3, the voltages flowing on the high-voltage lines 22 and 20 measured at a measurement location 30 apart by about 5 cm from the output channels CH1 to CH4 (shown in FIG. 1) has also a deviation due to a length difference between the first and the second high-voltage lines 22 and 20.
Accordingly, a deviation of currents and voltages occurs in the AC waveforms supplied to drive the lamps 24, the first and second electrodes of each the lamps 24 receive different waveforms. As a result, a deviation in panel brightness is generated, to thereby induce an uneven brightness phenomenon in the panel.